Voltage overshoot protection

ABSTRACT

A transmission circuit for an RF inductive link, particularly for an implanted device such as a cochlear implant. In a preferred form, the transmission circuit  1  includes a transmitter coil  24  and a damping device, including an auxiliary coil  4  and switch  6 . The switch is operated to close the coil circuit when data zeros are transmitted. This has the advantage of improving modulation depth without placing stress on the RF driver output switches.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/196,533, entitled “Voltage Overshoot Protection,” filed on Aug. 2, 2011, which is a continuation of U.S. patent application Ser. No. 12/625,356, entitled “Voltage Overshoot Protection,” filed on Nov. 24, 2009, now U.S. Pat. No. 7,991,335 issued Aug. 2, 2011, which is a continuation of U.S. patent application Ser. No. 10/968,001, filed Oct. 20, 2004, now U.S. Pat. No. 7,623,827, issued on Nov. 24, 2009, which claims priority from Australian Provisional Patent Application No. 2003905729 filed on Oct. 20, 2003. The entire disclosure and contents of the above applications are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to inductive data links, and particularly but not exclusively to data links and transmission protocols that are used in conjunction with implantable medical devices.

2. Related Art

Inductive links are used in many applications, but have particular application to implanted medical devices such as pacers, cochlear implants and other neural stimulators, and other types of devices. They allow for data and/or power to be transmitted to a device which is wholly implanted from an external system, without requiring a cable or connection to pass through the skin.

Hearing aids and cochlear implants are useful in restoring the sensation of hearing to hearing impaired individuals. This is an area where inductive links are widely applied.

A cochlear implant is used where the hair cells of the cochlea have been damaged to the extent that they are no longer able to convert the mechanical vibration of the cochlea fluid into an electrical signal. The cochlear implant generally bypasses the hair cells of the cochlea and delivers electrical stimulation, representative of speech and environmental sounds, to the nerves in the cochlear. The neural impulses generated by the electrical stimulation are then sent to the brain, thereby allowing the brain to perceive a hearing sensation.

The functional components of a cochlear implant system typically include an external sound processor device connected to an external headset/transmitter coil, and an implanted receiver-stimulator connected to an implanted electrode array.

In operation, the sound processor device detects sound and converts this into a binary coded, serial data stream, representative of the desired stimulation parameters. The serial data stream then modulates a high frequency (RF) analogue carrier. Typically, the serial data stream is 100% amplitude modulated on the analogue carrier, for example as is described in U.S. Pat. No. 4,532,930 (“Crosby”) or U.S. Pat. No. 5,741,314 (“Daly”). The contents of these documents are incorporated herein by reference.

The modulated signal is then output from a number of RF output driver circuits in the speech processing device, and sent to the headset/transmitter coil.

The headset/transmitter coil is frequency-tuned as a series resonance circuit that is inductively linked with a parallel-tuned resonant circuit in the form of the receiver coil in the receiver-stimulator. A voltage waveform thus induced in the receiver coil has a voltage waveform envelope representing the serial data steam.

A problem with such inductive communication links is that errors can occur when the modulation depth of the waveform received in the receiver coil falls below a certain threshold.

The modulation depth of the received waveform depends on a number of factors.

One factor is the tuned frequency and quality factor of the transmitter and receiver coils, since the higher the effective quality factor, the lower the modulation depth of the received waveform and the longer the after-burst ringing in both the transmitted and the received waveforms.

Another factor is the effective coupling coefficient between the transmitter and receiver coils. The modulation depth of the received waveform decreases with an increased distance between the two coils.

The prior art has employed a ‘tri-state’ function to reduce such data errors. Basically, this involves open-circuiting the output switches of the RF drivers during transmission of data “zeros”, inter-phase gaps and/or inter-frame gaps. Conversely, the output switches of the RF drivers are close-circuited during transmission of data “ones”. This ‘tri-state’ function reduces the effective quality factor of the transmitter coil, increases the resonance frequency, thus creating a higher frequency post RF-burst ringing that decays quickly. The interaction of the transmitted signal with the receiver coil leads to a larger modulation depth and thus improved data integrity.

However, a drawback of the prior art tri-state function is that an excessive voltage overshoot is generated when the output switches of the RF drivers are open-circuited. For example, RF switches operating with a supply voltage of 3.3V will produce an RF output voltage swinging between −0.8V and 4.1V, as a result of the output voltage overshoot when the switches are opened. The voltage of the RF drivers exceeds the power supply rails by a forward diode drop, typically about 0.8V above VDD and 0.8V below VSS.

This voltage overshoot can affect the reliability of the integrated circuit since the output switches may be overstressed from being operated outside the maximum operating voltage specified by the integrated circuit manufacturer. Further, this overstressing can gradually weaken, and eventually permanently damage the output switches.

One technique used to reduce the effects of this voltage overshoot is to provide protection diodes at each of the output switches. However, these diodes only share the current during the voltage overshoot and do not affect the voltage by any significant amount. Further, voltage waveform clamping and current conduction through the diodes defeats the purpose of the tri-state function until the signal drops below the clamping point.

It is an object of the present invention to provide an improved RF transmission arrangement for inductive links.

SUMMARY

In one aspect of the present invention, there is provided an RF transmission circuit for transmitting at least a data signal across an inductive link. This transmission circuit comprises modulation means for modulating a high frequency carrier signal with the data signal to provide an output signal; and transmission means, including a transmission coil, for transmitting the output signal across the inductive link, wherein said transmission means includes an output signal damping means that is selectively operable according to the value of the data signal.

In another aspect, there is provided an RF transmission circuit for a cochlear implant system. This transmission circuit comprises modulating means for modulating a high frequency carrier signal with a serial data stream for transmission to an implanted stimulation device; and a headset coil for transmitting an output signal from said modulating means across a transcutaneous inductive link for detection by an implanted receiver coil; wherein said circuit further comprises a damping means that is selectively operable according to a momentary value of the serial data stream.

In yet another aspect, there is provided a method of transmitting a stream of binary data across an inductive link. This method comprises: modulating a high frequency carrier signal with the data to provide an output signal; and transmitting said output signal across the inductive link; wherein said transmitting step includes the sub-step of selectively damping said high frequency carrier signal according to a momentary value of the binary data.

In yet another aspect, there is provided an RF transmission circuit for transmitting at least a data signal across an inductive link. This transmission circuit comprises: a modulator configured to modulate a high frequency carrier signal with the data signal to provide an output signal; a transmission coil configured to transmit the output signal across the inductive link; and a damping arrangement configured that is selectively operable according to the value of the data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is an illustrative diagram showing a general arrangement of a cochlear implant system;

FIGS. 2A to 2C show explanatory example waveforms used to assist in the understanding of this disclosure;

FIG. 3 is a further explanatory example waveform;

FIG. 4 is a schematic diagram of a transmitter circuit according to this disclosure; and

FIGS. 5A to 5F show a series of comparative waveforms used to assist in the understanding of this disclosure.

DETAILED DESCRIPTION

It will be appreciated that whilst present invention will be principally described with reference to cochlear implants, the principles of the present invention may be applied in relation to many other types of inductive links, and in particular to other types of implanted medical devices. It is applicable whether or not power and data, or data alone, are being transmitted. It is also applicable whether the link operates more or less continuously, or in a more sporadic or occasional mode.

Before describing the features of one implementation of the present invention in detail, it is convenient to further describe the construction and overall operation of one example of a cochlear implant system. It will be understood that the present invention could be applied to other types of cochlear implants.

Referring to FIG. 1, the external sound processor device 29 includes an on-board microphone 27 and is generally configured to fit behind the outer ear 11, as shown. The sound processor device 29 is connected to a headset transmitter coil 24 which transmits power and data signals to the implanted receiver-stimulator unit 22 over a transcutaneous RF link.

The signals transmitted by the headset coil 24 are received by a receiver coil 23 in the receiver-stimulator unit 22 and then applied to the electrode array 20. The electrode array 20 applies electrical stimulation signals to the basilar membrane 8, to thereby stimulate the auditory nerve 9. The electrical stimulation signals are normally bi-phasic and charge balanced to ensure there is no net DC current flow.

It is noted that the receiver coil 23 can also transmit signals back to the headset coil 24 for telemetry purposes, as described for example in U.S. Pat. No. 5,758,651, the contents of which are incorporated herein by reference.

The sound processor device 29 receives sound and calculates a digital data stream, representing the electrical stimulation parameters according to a selected coding strategy. The digital data stream then modulates a radio frequency (RF) carrier which is then inductively transmitted over the transcutaneous link to the receiver coil 23. Typically, the means for modulation performs 100% amplitude modulation and can be combined with pulse width modulation, as shown for example in FIG. 2A.

In this example, the RF signal is transmitted at an average signal frequency of 5 MHz, although the headset transmitter coil 24 and the receiver coil 23 are stagger-tuned to achieve the required bandwidth for the data communication. For example, the headset coil 24 is tuned to 4.8 MHz in a high quality-factor, series resonance circuit and the receiver coil 23 is tuned to 5.2 MHz in a parallel resonance circuit.

The headset coil 24 and receiver coil 23 are separated by a distance of between approximately 1 mm and 10 mm. FIGS. 2B and 2C show how the waveform received at the receiver coil 23 can vary according to the separation distance between the headset coil 24 and the receiver coil 23.

The voltage waveform across the receiver coil 23 is an amplitude modulated signal whose envelope represents the original serial digital data steam. A typical modulated signal has a series of 5 pulses for a one, and 5 time spaces with no pulses for a zero.

The receiver-stimulator unit 22 receives and decodes the amplitude modulated signal into a stimulation signal which is then applied to the electrode array 20. The incoming signal is also rectified to generate an internal power supply.

Referring now to FIG. 4, an example of a transmitter circuit 1 according to the present disclosure will be described.

The transmitter circuit 1 includes the headset transmitter coil 24 as described above, an RF driver circuit 2 and an auxiliary coil 4. The RF driver circuit 2, which is normally provided as an integrated circuit, includes a number of H-bridge RF output driver switches 5 and a low impedance switch 6 connected to the auxiliary coil 4. Preferably, the auxiliary coil 4 has a single turn and is provided on a printed circuit board (PCB) in common with the transmitter coil 24.

It is preferred that the auxiliary coil 4 is physically located in close proximity to the transmitter coil 24, and is dimensioned so as to have as high a coupling co-efficient with the transmitter coil 24 as possible. It may conveniently be located within the headset with the transmitter coil.

In operation, when the transmitter coil 24 transmits a data “1”, the low impedance switch 6 is open circuited, and thereby inhibits current flow through the auxiliary coil 4.

Conversely, when the transmitter coil 24 transmits a data “0”, the low impedance switch 6 is closed and the auxiliary coil 4 is short-circuited. The low impedance switch 6 is also closed during inter-phase gaps and inter-frame gaps.

The switching may be controlled by detection of data zeros in the data stream. Alternatively, a separate signal could be sent by the speech processor to control switching.

When the auxiliary coil 4 is short-circuited, its inductance is reflected as a high capacitive impedance, in series with the transmitter coil 24. This causes a large increase in the tuned frequency of the transmitter coil 24, which when combined with the losses of the auxiliary coil 4 and the low impedance switch 6, leads to fast decay or damping of the post RF-burst ringing waveform. In effect, the effective inductance of the transmitter circuit is increased. A fast decay of the ringing waveform improves the modulation depth and hence reliability of the data signal against errors, as earlier discussed. The RF driver switches 5 can operate normally, without subjecting them to excessive voltage overshoots.

The low impedance switch 6 is controlled by a signal that causes the switch 6 to close according to the data signal. Accordingly, the switch 6 is closed when the momentary value is binary zero, during interphase gaps and during interframe gaps.

An example of a digital serial data stream is shown as the upper waveform in FIG. 3, whereas the lower waveform of FIG. 3 shows a control signal applied to the low impedance switch 6.

The coupling coefficient K0 between the transmitter coil 24 and the auxiliary coil 4 is usually set to a high value, for example, greater than 70%. The inductance of the auxiliary coil 4 is designed so that the peak voltage induced across the auxiliary coil 4 cannot exceed one diode drop above VDD, to avoid signal clamping.

It can be seen that the transmitter circuit 1 thus provides signal damping without causing excessive voltage stress on the RF output switches 5, as is the case with the earlier tri-state function. The integrated circuit can be protected against voltage overshoot of the resonance circuit associated with the transmitter coil 24, without compromising the data integrity of the RF link.

The RF output remains connected to either VDD or VSS, depending on the circuit design and the polarity of the input clock, as will be understood by the person skilled in the art.

FIGS. 5A to 5F show a series of waveforms demonstrating test results of the transmission circuit 1 according to this disclosure, in comparison with prior art, tri-stating techniques.

FIG. 5A labeled “RF Driver output voltage” shows two waveforms, side by side. The waveform of the left hand side of FIG. 5A shows a voltage overshoot associated with the prior art tri-state function, when the RF output driver is open circuited after 2.0 μs. In contrast, the waveform on the right hand side of FIG. 5A does not have the same voltage overshoot because a reflected load is applied in accordance with this disclosure, after 2.0 μs.

Similarly, FIG. 5B labeled “Transmitter coil current” shows on the right hand side how an absence of voltage overshoot reduces the number of ringing cycles during a zero data interval.

FIGS. 5C and 5D also show comparative waveforms for the implant voltage and implant VDD, respectively.

FIGS. 5E and 5F show the current waveforms in the transmitter coil 24 and auxiliary coil 4 respectively. The switch is operated just before time 6.0 microseconds on each graph, and shows the rapid decay of ringing in the transmitter coil.

Although the present invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is merely illustrative of the application of the principles of the invention. Thus numerous modifications may be made as will be understood by those skilled in the art, without departing from the principles of the invention. 

1. An RF transmission circuit for transmitting a data signal, said transmission circuit comprising: a transmission coil assembly configured to transmit the data signal across an inductive link; and a signal generator configured to selectively output high frequency signal pulses to the transmission coil assembly during a series of time spaces to represent data, wherein the RF transmission circuit is configured to increase an effective inductance of the RF transmission circuit when the signal generator is not outputting the high frequency signal pulses to the transmission coil assembly.
 2. The RF transmission circuit of claim 1, wherein: the RF transmission circuit is configured to decrease the effective inductance of the RF transmission circuit when the high frequency signal pulses are outputted to the transmission coil assembly.
 3. The RF transmission circuit of claim 1, wherein: the signal generator is configured to output the high frequency signal pulses to the transmission coil assembly during a time space to represent one digit of a binary digit set.
 4. The RF transmission circuit of claim 3, wherein: the RF transmission circuit is configured to not output the high frequency signal pulses to the transmission coil assembly during a time space to represent the other digit of the binary digit set.
 5. The RF transmission circuit of claim 1, further comprising: a damping arrangement configured to be selectively operable during the series of time spaces, wherein the RF transmission circuit operates the damping arrangement during the time spaces when the signal generator is not outputting the high frequency signal pulses to the transmission coil assembly.
 6. The RF transmission circuit of claim 5, wherein: the RF transmission circuit does not operate the damping arrangement during the time spaces when the signal generator is outputting the high frequency signal pulses to the transmission coil assembly.
 7. The RF transmission circuit of claim 1, wherein: the transmission coil assembly includes a primary coil and an auxiliary coil, the RF transmission circuit is configured to inhibit current flow through the auxiliary coil during the time spaces when the signal generator is outputting the high frequency signal pulses to the transmission coil assembly; and the RF transmission circuit is configured to enable current flow through the auxiliary coil during the time spaces when the signal generator is not outputting the high frequency signal pulses to the transmission coil assembly.
 8. The RF transmission circuit of claim 1, wherein: the transmission coil assembly includes a primary coil and an auxiliary coil, the RF transmission circuit is configured to establish an inductance of the auxiliary coil as high capacitive impedance in series with the primary coil during the time spaces when the signal generator is not outputting the high frequency signal pulses to the transmission coil assembly; and the RF transmission circuit is configured to cancel the established inductance of the auxiliary coil during the time spaces when the signal generator is outputting the high frequency signal pulses to the transmission coil assembly.
 9. The RF transmission circuit of claim 1, wherein: increasing the effective inductance of the RF transmission circuit when the signal generator is not outputting the high frequency signal pulses to the transmission coil assembly increases a decay time of a ringing waveform existing in the RF transmission circuit from the high frequency signal pulses outputted to the transmission coil assembly.
 10. An apparatus for transmitting a data signal across an inductive link, said apparatus comprising: a transmission coil; and an RF data transmission circuit configured to: provide to the transmission coil high frequency signal pulses during selected time spaces of a series of time spaces; and transmit high frequency transmitted signal pulses from the transmission coil during the selected time spaces based on the received high frequency signal pulses having a first power; and reduce a power of any high frequency signal pulses transmitted by the RF transmission circuit resulting from post RF-burst ringing during non-selected time spaces, the post RF-burst ringing resulting from the high frequency signal pulses having a first power received during the selected time spaces.
 11. The apparatus of claim 10, further comprising: a damping arrangement, wherein the RF data transmission circuit is configured to selectively operate the damping arrangement during the non-selected time spaces, thereby reducing the power of any high frequency signal pulses transmitted by the RF transmission circuit resulting from post RF-burst ringing.
 12. The apparatus of claim 11, wherein: the RF data transmission circuit is configured to selectively increase an effective inductance of the RF data transmission circuit during the non-selected time spaces.
 13. The apparatus of claim 10, wherein: the RF data transmission circuit is configured to provide the high frequency signal pulses having a first power to the transmission coil during the selected time spaces to represent one digit of a binary digit set.
 14. The apparatus of claim 10, wherein: the RF data transmission circuit is configured to withhold high frequency signal pulses from the transmission coil during the non-selected time spaces to represent the other digit of the binary digit set.
 15. The apparatus of claim 11, wherein: the RF data transmission circuit is configured to deactivate the damping arrangement during the selected time spaces.
 16. The apparatus of claim 10, wherein: the transmission coil assembly includes a primary coil and an auxiliary coil, the RF transmission circuit is configured to establish an inductance of the auxiliary coil as high capacitive impedance in series with the primary coil during the selected time spaces; and the RF transmission circuit is configured to cancel the established inductance of the auxiliary coil during the non-selected time spaces.
 17. A medical prosthesis configured to transmit a data signal across a transcutaneous inductive link, said prosthesis comprising: an external component including: a transmission coil assembly; and an RF data transmission circuit configured to: transmit high frequency signal pulses having a first power from the transmission coil during selected time spaces of a series of time spaces; and reduce a power of any high frequency signal pulses transmitted by the RF transmission circuit during non-selected time spaces to below a second power through signal pulse damping; and an implantable component including: an RF receiver circuit, wherein the second power corresponds to a power below a minimum detection level threshold of the RF receiver circuit, and wherein the first power corresponds to a power above the minimum detection level threshold of the RF receiver circuit.
 18. The medical prosthesis of claim 17, wherein: the signal pulse damping is achieved by increasing the effective inductance of the RF transmission circuit from that of the RF transmission circuit during the selected time spaces.
 19. The medical prosthesis of claim 17, wherein: the transmission coil assembly includes a primary coil and an auxiliary coil, the RF transmission circuit is configured to inhibit current flow through the auxiliary coil during the selected time spaces; and the RF transmission circuit is configured to enable current flow through the auxiliary coil during the non-selected time spaces.
 20. The medical prosthesis of claim 17, wherein: the transmission coil assembly includes a primary coil and an auxiliary coil, the RF transmission circuit is configured to establish an inductance of the auxiliary coil as high capacitive impedance in series with the primary coil during the non-selected time spaces; and the RF transmission circuit is configured to cancel the established inductance of the auxiliary coil during the selected time spaces. 